The present invention relates to a transfer device provided with a guide member for restricting a conveyance direction of a recording material and to an image forming apparatus.
Among image processing apparatus, there are ones that are configured so as to transfer a toner image formed in an image forming section onto a recording paper sheet by a transfer device and then so as to guide the recording paper sheet to a fuser unit with a paper guide. Among such kind of image forming devices, there have conventionally been ones in which in transfer processing a stain is removed from a surface of a transfer belt by a cleaner (cleaning blade) provided in the transfer device, because unnecessary toner such as fog toner sticks to the transfer belt in transfer processing (for instance, refer to Japanese Patent Unexamined Publication No. 2007-94343 bulletin).
However, in such a conventional image forming apparatus as described in the Japanese Patent Unexamined Publication No. 2007-94343 bulletin, since the cleaner is always kept in contact with the transfer belt, there have been problems that the transfer belt elongates with the passage of time, thereby resulting in the occurrence of wrinkles and/or meandering, and/or that the transfer belt is damaged.
Then, in order to solve such problems, it is possible to conceive an image forming apparatus in which a transfer device, which is not provided with a cleaner, causes unnecessary toner to adhere from the secondary transfer belt to a primary transfer belt and then to be collected by a cleaner for the primary transfer belt. This sort of image forming apparatus has, for example, a configuration like an image forming apparatus 200 shown in FIGS. 1, 2.
In transfer processing, as shown in FIG. 1, the image forming apparatus 200, by causing a switch 621 to change over, connects a first power supply 622 to a primary transfer belt drive roller 62, and causes a first electric current (for instance, minus 40 microamperes) to flow from a primary transfer belt drive roller 62 to a secondary transfer roller 312. By this procedure, a difference in electric potential occurs between a primary transfer belt 6 and a paper sheet P, so that a toner image that is borne on the primary transfer belt 6 undergoes a secondary transfer onto the paper sheet P due to an electrostatic force.
In cleaning processing, as shown in FIG. 2, the image forming apparatus 200, by causing the switch 621 to change over, connects a second power supply 623 to the primary transfer belt drive roller 62, and causes a second electric current that is smaller in absolute value than the one in transfer processing to flow from the primary transfer belt drive roller 62 to the secondary transfer roller 312. Because most part of the unnecessary toner on the secondary transfer belt 310 is fog tonner that has a small quantity of electrostatic charges, a moderate difference in electric potential occurs between the primary transfer belt 6 and the secondary transfer belt 310 when the second electric current (for instance, minus 10 microamperes) that is smaller in absolute value than the one in transfer processing is caused to flow; so that the unnecessary toner on the secondary transfer belt 310 adheres to the primary transfer belt 6 due to an electrostatic force, and then the unnecessary toner is collected by the primary transfer belt cleaning unit (not shown).
Therefore, in the image forming apparatus 200, neither a wrinkle, damage nor the like occurs to the secondary transfer belt 310; also, unnecessary toner can be removed from the secondary transfer belt 310.
However, in the image forming apparatus 200, because of the three possible causes as described below, there is a problem that in cleaning processing for the secondary transfer unit 31 an electrical discharge noise occurs between the secondary transfer belt drive roller 313 and a pre-fusing guide member 18 due to an electrical discharge phenomenon.
Cause 1: The secondary transfer belt drive roller 313 in an electrically floated state (a floating state) becomes a high voltage (for instance, 2.2 kV).
Cause 2: The pre-fusing guide member 18 is arranged in the vicinity (for instance, a distance of about 1 mm through 3 mm) of the secondary transfer belt drive roller 313.
Cause 3: The pre-fusing guide member 18 is grounded.
That is, in cleaning processing, since the drive roller 313 is at a high voltage while the grounded pre-fusing guide member 18 is installed in the vicinity of the drive roller 313, the electrical discharge phenomenon occurs between the drive roller 313 and the guide member 18.
As to the Cause 1, the reason why the secondary transfer belt drive roller 313 becomes a high voltage is as follows. That is to say, it is thought to be caused by the electric charges accumulated in excess onto the roller 313 through the secondary transfer belt 310 because an electric current value flowing to the roller 312 is small due to the fact that the second electric current value (absolute value) is small and that the secondary transfer roller 312 has a resistance, and further because the secondary transfer belt drive roller 313 is caused to be in an electrically floated state. When the absolute value of the second electric current value is increased (so as to become close to the first electric current value) in order to avoid the roller 313's state of reaching a high voltage, the electric current value flowing to the secondary transfer roller 312 also increases; consequently, while the voltage of the roller 313 is lowered as the excessive accumulation of electric charges onto the roller 313 ceases to occur, the fog toner on the secondary transfer belt 310 is charged again, thereby being rendered unable to be completely removed. Therefore, the above described second electric current value cannot be increased. After all, Cause 1 cannot be solved.
As to the Cause 2, when the pre-fusing guide member 18 is installed being separated from the secondary transfer belt drive roller 313, there arises a risk that the paper sheet P is rolled up onto the secondary transfer belt 310. Therefore, the Cause 2 is also difficult to solve.
As to the Cause 3, when the pre-fusing guide member 18 is fixed in an electrically floated state (the first countermeasure), it is possible to prevent the aforementioned electrical discharge phenomenon from occurring in cleaning processing. In that case, however, in transfer processing (image forming processing), since remnant electric charges are accumulated onto the pre-fusing guide member 18, image deletion occurs to the toner image on the paper sheet P due to the excessive electrostatic charges that have accumulated, for example, in the course of double sided copying performed onto about 200 paper sheets. Therefore, the Cause 3 is also difficult to solve.
With regard to the Cause 3, the image deletion can be prevented from occurring by having the secondary transfer belt drive roller 313 grounded in addition to having the pre-fusing guide member 18 grounded (the second countermeasure). In that case, however, in transfer processing, since the paper sheet P is diselectrified twice, i.e. first on passing the neighborhood of the secondary transfer belt drive roller 313 and then on touching the pre-fusing guide member 18, an electrostatic adsorption force acting on the toner toward the paper sheet P decreases to a large extent. Therefore, the toner moves on the paper sheet P, thereby resulting in the occurrence of a picture quality disorder.
In this manner, with the image forming apparatus as shown in FIGS. 1, 2, in cleaning processing, facing the problem that the electric discharge noise occurs between the secondary transfer belt drive roller 313 and the pre-fusing guide member 18 is unavoidable. Also, there is a problem that the above described first and/or second countermeasures to avoid the above problem can cause the image deletion and/or the picture quality disorder.
Thus, the present invention is principally directed to providing a transfer device free from an electric discharge noise.
The present invention is also directed to providing a transfer device free from an image deletion and/or a picture quality disorder in a toner image on a paper sheet.
The present invention is further directed to providing an image forming apparatus using the transfer device.